Typical applications for focused ion beam (FIB) systems are the preparation of TEM lamellae, cross-sections, or samples suitable for 3D tomography. All these applications involve the excavation of a trench out of the sample bulk material.
Preferably, the cross-section or TEM lamella should be observable while being prepared. To that end, combined SEM-FIB-systems are used. Such systems include two particle-optical columns e.g. an electron column (SEM) and an ion column (e.g. a FIB column), whereby both columns are arranged in a particular angle to each other. This arrangement allows processing the sample by milling with the ion beam as well as observing the process via SEM.
A cross-section is a polished surface perpendicular to the sample surface. In order to create a cross-section of a given width and depth, a volume of sample material is removed by milling with the FIB. This will not only prepare the cross-section as such but also provide a viewing path for the SEM. Depending on the angle between SEM and FIB, the trench should feature a slope with the base of the slope at the bottom part of the cross-section. The sloped trench will allow the user to observe the entire cross-section. The desired angle of observation determines how much material has to be removed when preparing the sloped trench. The viewing angle does not always have to be the angle between the SEM column and the FIB column. Instead, sometimes steeper angles are chosen to reduce working distance (WD) or minimize material removal.
Similarly, TEM lamellae are prepared for lift-out by removing a volume of material from two opposing sides of the sample region of interest. This means that in principle two cross-sections have to be prepared, one at each side of the TEM lamella to be prepared. The amount of material to be removed depends on the desired shape of the lamella and the restrictions given by the accessible cutting angles for detaching the bottom of the lamella.
When excavating a trench out of the sample material, usually relatively large amounts of sample material have to be removed which is often a time-consuming procedure. Therefore, the feasibility of a particular investigation is often determined by whether the material removal can be accomplished within a reasonable time.
Thus, milling speed can be an important factor for the preparation of samples by ion beam milling.
Milling speed depends on a multitude of factors such as sample material, lattice orientation, ion current, ion species and/or milling geometry. Besides that, redeposition processes can play an important role. During milling, sample material is sputtered away from the sample, but a certain amount of the removed material is deposited onto the sample surface again. Often, redeposited material fills up most of the recently prepared trench, so that trenching speed is considerably reduced. Thus, the sputtering yield, i.e. the material removal rate, could be much higher with reduced or even without redeposition.
Usually, milling is done by processing individual milling objects. With the help of milling objects, which are geometrical patterns e.g. rectangles, the user can select the area to be scanned by the focused ion beam, thus defining the sample region to be milled. The milling objects are shown in the user interface of the machining system (FIB system or combined FIB-SEM-system), typically superimposed on a microscopic image of the sample surface.
The scanning strategy, i.e. the manner how the ion beam is guided over the sample region to be milled, has a strong impact on the achievable material removal rate. Currently, there are two established milling modes for coarse material removal: line milling and frame milling.
In line milling mode, a single line is scanned several times by scanning the ion beam forth and back along the line until the full ion dose for this region has been delivered. Then, the ion beam jumps to the next line, which is scanned in the same manner. Thus, the desired ion dose is delivered to each line in a single pass, which itself includes a plurality of forth and back passes along the same line. The milling is completed when the ion beam has scanned the last line of the respective milling object. With line milling mode relatively high removal rates are obtainable due to the edge geometry.
Typically, the sputtering yield of line milling mode is six to eight times higher than that of frame milling mode. With line milling mode relatively deep trenches can be cut, creating a smooth cross-section at the lower end of the sloped trench. However, due to redeposition occurring in previously excavated regions of the trench, the observable area is limited by piled-up redeposition material. Thus, disadvantageous with line milling mode is, that it is difficult to generate a trench with a specific slope angle. Sometimes multiple line milling objects are superimposed and processed sequentially to remove redeposition and achieve the desired geometry.
In frame milling mode, the beam is scanned over the entire milling region in a first pass. As soon as a line has been scanned once, the ion beam jumps to the next line and so on until the frame is completed. When the frame is completed, which means that all lines of the frame have been scanned once, the beam jumps back to the first line of the frame and the process is repeated in a further pass. Therefore, during each pass only a small fraction of the total ion dose is applied to the sample material. The milling process is completed when the desired total ion does has been applied.
When using frame milling mode to prepare a trench for cross-sectioning, usually several milling objects are superimposed to approximate the desired trench slope. This leads to a staircase form (staircase milling). Since each milling object is scanned several times during exposure, in each pass redeposition from the previous pass is removed. However, because of the short dwell time for each beam position on the sample, sputtering yield is remarkably lower than for line milling. Moreover, redeposition—despite not being visible—reduces the effective sputtering rate as the aspect ratio increases.
However, the distinction between frame milling and line milling is by convention. Very slow frame scan (long dwell time) or very narrow line spacing (smaller than the beam diameter) makes frame mode and line mode similar.
Overall, neither the known procedures for line milling mode nor those for staircase milling make optimal use of the applied ion dose.
It is known in the art to use a combination of coarse milling and fine milling, i.e. different milling currents, in order to prepare a cross-section. Initially, a high current is used to remove an amount of material. Then, a significantly lower current is applied for cleaning or polishing the cuts while employing a smaller beam diameter (i.e. a finer probe).
By way of example, in U.S. Pat. No. 7,427,753, a sequence of coarse scanning and fine scanning is suggested using a line-interlace mode by creating sub-groups of milling lines.
Moreover, Adams, D. P; Vasile, M. J. (2006): J. Vac. Sci. Technol B 14(2), March/April 2206, p 836-844 and Bassim, Nabil; Scott, Keana; Giannuzzi, Lucille A.: MRS Bulletin, Vol 39, April 2014, p. 317-325 suggest using a boustrophedonic frame scanning mode, wherein the pixel dwell time is varied (Adams & Vasile (2006)).
Another strategy to accelerate milling speed is to increase the milling current, i.e. the beam current used for milling. This can be done for example by using a plasma FIB. However, increasing the milling current also increases the beam (i.e. probe) diameter, leading to a decrease of accuracy.